The invention relates to methods for providing a barrier metal between an aluminum metalization layer and the silicon surface of a semiconductor wafer.
One of the difficulties in obtaining high yields of bipolar LSI and VLSI integrated circuits is in obtaining good quality thin films for the standard two or more layer metallization schemes. One of the most serious problems has to do with making sure that these films are able to adequately cover topographical steps on the silicon wafer surface and maintain integrity as they do so. By far, the metal element most commonly used in the semiconductor industry for metal interconnects is aluminum. Unfortunately, aluminum has an affinity for silicon during the sintering process which results in the aluminum signal line pitting through and shorting out shallow junctioned bipolar devices. The answer to this problem has been to utilize aluminum alloys containing silicon. While this solves the pitting problem, it causes difficulties with thin film step coverage. When the aluminum silicon layer has been etched to define the metalization interconnect pattern, the silicon in the alloy is left behind in the form of small nodules. These nodules serve as nucleation centers during subsequent dielectric chemical vapor deposition and produce a very rough surface and even bulbs at steps, resulting in a surface which is very difficult to cover with another layer of metal.
A solution to this problem has been to use non-silicon containing alloys of aluminum together with a barrier metal between the aluminum and the silicon to prevent alloy pitting. A very good candidate for this barrier metal is titanium-tungsten alloy. Unfortunately, titanium-tungsten makes good ohmic contact to N type silicon but not to P type. Past process utilizing titanium-tungsten have, therefore, necessitated the use of an extra photolithographic masking operation, making the process more complex and costly. That is, in the past, the titanium-tungsten layer had to be defined by photolithographic means and then etched from the P type contact windows prior to depositing the aluminum metalization layer.